An ecosystem model is a tool to monitor the effects of managment strategies on the ecosystem functioning. In OMES the further development an existing ecosystem-model (MOSES) of the Sea Scheldt is the priority. In the OMES project already a lot of chemical, biological and ecological data were gathered to improve the knowledge of the ecosystem of the Scheldt estuarine. To increase insight, experimental project were established at Kruibeke and Lippenbroek to monitor and simulate environmental conditions in controlled flooding area's. An integrated and coordinated monitoringsprogram is nevertheless necessary to develop further future research on the functioning of the estuarine ecosystem of the Scheldt. The 4th phase of OMES continues the monitoring of the different ecosystem components, but will give more attention to the possible role of flooding areas that can be used as protection strategy against floods, especially to the high value of these areas as ecological important habitats. The following studies, concerning the different components of the estuarine ecosystem, will be executed: Fourth phase, 2006-2007 The basic water quality in the Flemish part of the Scheldt estuary The C cycle of the Flemish part of the Scheldt estuary Primary production in the Flemish part of the Scheldt estuary Sedimentology of the Flemish part of the Scheldt estuary The phytoplankton of the Flemish part of the Scheldt estuary The zoˆplankton of the Flemish part of the Scheldt estuary Study on the vegetation under tidal range in the Flemish part of the Scheldt estuary: 2 mesocosmos experiments and experimental area Lippenbroek details 'presentation' Evaluation and update of the OMES database, concerning the data of the Flemish part of the Scheldt estuary details